Membrane electrolysis processes of industrial interest such as chlorine and caustic soda production from sodium chloride solutions and even more for the production of chlorine from hydrochloric acid solutions or directly from gaseous hydrochloric acid as described in U.S. Pat. No. 5,411,641, J. A. Trainham III, C. G. Law Jr, J. S. Newman, K. B. Keating, D. J. Eames, E. I. Du Pont de Nemours and Co. (USA), May 2, 1995, undergo extremely aggressive conditions.
In the process for the production of caustic soda and chlorine, the anodic reaction produces chlorine gas which, as is well known, is a strongly corrosive agent. For this reason, in industrial practice usually titanium is used for the anodic elements of the elementary cells forming the electrolyzers. The use of titanium, in this case, is permitted by the relatively modest acidity of the sodium chloride brine in contact with said anodic parts. The acidity is kept at low levels for process reasons and mainly not to damage the delicate ion-exchange membranes separating with a high efficiency the produced caustic soda from the acid brine Suppliers of this kind of membranes specify in fact that the minimum pH for continuous operation must be kept around 2.
Titanium cannot be used for the construction of the cathodic parts of the elementary cells forming the electrolyzer, as the hydrogen evolution, which is the only cathodic reaction, would cause a dramatic embrittlement. In most cases the cathodic parts of the elementary cells are made of high-alloy stainless steels or even better nickel. As a consequence, in bipolar electrolyzers, the bipolar elements which coupled together in a filter-press arrangement form the elementary cells, are made of two layers made of nickel and titanium connected either mechanically (U.S. Pat. No. 4,664,770, H. Schmitt, H. Schurig, D. Bergner, K. Hannesen, Uhde GmbH, May 12, 1987) or by welding (U.S. Pat. No. 4,488,946, G. J. E. Morris, R. N. Beaver, S. Grosshandler, H. D. Dang, J. R. Pimlott, The Dow Chemical Co., Dec. 18, 1984), optionally with an internal layer directed to ensure the electrical conductivity and necessary rigidity. These bipolar elements obviously entail a complicated construction and therefore high costs.
In the production of chlorine by electrolysis of hydrochloric acid, the aggressivity is much greater due to the concurrent presence of chlorine and high acidity. Under particular conditions (temperature below 60.degree. C., acid concentration below 20%, addition of passivating agents) a titanium--0.2% palladium alloy (ASTM B265, Grade 7) may be used with the interstice areas suitably protected by a proper ceramic coating. With temperatures and acid concentrations higher than the above mentioned ones and in the absence of passivating agents, the only suitable material for the construction of the anodic parts of the electrolyzer is tantalum, an extremely expensive material which poses a lot of problems for its working.
Anyway, tantalum, just as titanium, is not compatible with hydrogen and therefore cannot be used for the cathodic parts. A possible solution is given by the nickel alloys of Hastelloy B.RTM. type, but they are very expensive and undergo corrosion during the shut-downs of the electrolyzers. To avoid this severe inconvenience, it would be necessary providing the electrolysis plants with polarization systems, which would make scarcely practical the whole construction.
A possible alternative is offered by graphite, which is sufficiently stable at the process conditions, both the anodic (chlorine evolution with minor quantities of oxygen, in the presence of chlorides and acidity), and the cathodic ones (hydrogen in the presence of caustic soda--chlor-alkali electrolysis--or in the presence of acidity electrolysis of hydrochloric acid). Therefore graphite may be used in the form of plates directly forming the elements which are then assembled in a filter press-arrangement to form the elementary cells of electrolyzers. In the case of bipolar electrolyzers the two faces of the same graphite plate actually act as the cathodic wall of one cell and the anodic wall of the adjacent cell. As graphite is intrinsically porous, the mixing of chlorine and hydrogen, caused by diffusion through the pores, may be avoided only making the graphite plates impermeable by means of processes comprising filling under vacuum of the pores with a liquid resin which is subsequently polymerized and makes the graphite plate more stiff and enhances its chemical resistance characteristics. Graphite plates of this type are currently used in the industrial process known as "Uhde-Bayer" process for the electrolysis of hydrochloric acid solutions. Impermeable graphite however is extremely fragile and is not deemed acceptable for most chlorine producers, especially in critical apparatuses such as electrolyzers for chlorine production.
An interesting alternative is disclosed by U.S. Pat. No. 4,214,969, R. J. Lawrance, General Electric Company, Jul. 29, 1980 directed to the production of plates made of graphite powder and thermoplastic fluorinated polymers. The product obtained by heating and pressing the powders mixture is a composite having a minimum or no porosity, exhibiting a suitable electrical conductivity. This last characteristic is obviously necessary as the plates must provide for an efficient electric current transmission to ensure a correct operation of the electrolyzers. The advantage of the graphite-polymer composite over impermeable graphite is its higher stiffness. In fact, the two requisites, stiffness and electrical conductivity, are contradictory as a higher stiffness involves a greater amount of polymer while a greater amount of graphite would be necessary to enhance the electrical conductivity. As a consequence, an optimized product must be a compromise between the two needs, a compromise which the above patent indicates to be a function of the production parameters, in particular pressure and temperature.
When the thermoplastic fluoropolymer is the polyvinylidenefluoride, such as Kynar.RTM. produced by da Pennwalt (USA), the best results in terms of electrical conductivity and stiffness (measured as resistance to bending) are obtained with contents of polymer in the range of 20-25% by weight. Obviously, a composite plate obtained as above illustrated and with the aforesaid material is intrinsically expensive.
A reduction of the total costs of an electrolyzer obtained by assembling in a filter press-arrangement several plates may be achieved by eliminating from each plate every external connection (threaded joints, pipes, gaskets) for the circulation of the electrolytes and withdrawals of the products. This simplified design certainly increases the operation reliability of the electrolyzers, in particular when operating under pressure. The elimination of the external connection requires that each plate be provided with suitable internal holes provided with suitable distribution systems, as described in details in U.S. Pat. No. 4,214,969. the multiplicity of plates of the filter-press electrolyzer must have all the holes matching in order to form longitudinal channels inside the electrolyzer structure. These channels (manifolds), which are connected to suitable nozzles positioned on one or both sides of the electrolyzer heads, provide for the internal distribution to the various elementary cells of the fresh electrolytes and for the withdrawal of the exhausted electrolytes and electrolysis products (for example chlorine and oxygen). Said channels longitudinally crossing the electrolyzer are therefore subjected to a remarkable electric potential gradient. Further, if both the fresh and the exhausted electrolytes have a sufficient electrical conductivity (hydrochloric acid, sodium chloride brine and caustic soda are highly conductive), then the channels are crossed by consistent electric current, the so-called shunt current, which represent an efficiency loss and cause electrolysis phenomena among the surfaces of the plates facing the channels.
These electrolysis phenomena produce substantially two negative effects, that is the reduced purity of the electrolysis products and the corrosion of at least part of the composite plate surfaces. As a matter of fact also the graphite particles forming the composite may undergo corrosion and be progressively worn out and converted into carbon monoxide and/or carbon hydroxide under the electrolysis conditions typical of said channels. As a consequence, the composite looses its major components and thus any mechanical solidity.
U.S. Pat. No. 4,371,433, E. N. Balko, L. C. Moulthrop, General Electric Company, Feb. 1, 1983, describes a method for reducing parasitic shunt currents and eliminating corrosion phenomena. This method foresees a particular profile of the manifolds in order to cause a fractionating of the electrolyte flow in small droplets (increase of the overall electrical resistance) housing particular gaskets inside the manifolds. Substantially the surface of the composite plates facing the manifold is internally lined with the gaskets and cannot get in contact with the electrolytes. However, in view of the fact that these gaskets have a complex geometry and are made of elastomeric fluorocarbon materials which must ensure a high chemical resistance, such as Viton.RTM. polyhexafluoropropylene rubber supplied by DuPont (USA), this method is very expensive and therefore scarcely applicable in industrial practice.